U.S. Pat. No. 2,684,331 discloses a method for chromatographic separation from one another two or more substances having widely different ionization constants and at least one of the substances undergoes conciderable ionization in a dilute aqueous solution thereof. However, the method has not been used for separating carbohydrates. The examples of the U.S. Pat. No. 2,684,331 describe separation of salts from organic solvents, e.g. sodium chloride from formaldehyde. The method comprises an ion exchange resin having an ion identical with an ion of highly ionized solute. The ion exchange resin is either a cation exchange resin or an anion exchange resin. The cation exchange resin contains sulphonate groups as functional groups. The anion exchange resin contains quaternary ammonium groups as functional groups.
U.S. Pat. No. 2,911,362 describes a method comprising a chromatographic separation process employing ion exchange resins for separating two or more water soluble organic compounds from one another in an aqueous medium in the absence of an ion exchange reaction, i.e. in the substantial absence of a chemical reaction involving an absorption of ions from the aqueous medium by the resin or the introduction of ions into the solution from the resin. According to said method the ion exchange resin can be either a cation exchange resin or an anion exchange resin. The cation exchange resin may contain either sulfonate groups as functional groups or carboxylic acid groups as functional groups. The anion exchange resin contains quaternary ammonium groups as the functional groups therein. However, the separation has not been used for separation of carbohydrates.
Chromatographic separation has been used for recovery of xylose from hydrolysates of natural materials such as birch wood, corn cobs and cotton seed hulls in a method described in U.S. Pat. No. 4,075,406. The resin employed in the chromatographic separation is a strongly acid cation exchanger, i.e. sulfonated polystyrene cross-linked with divinyl benzene. The use of a strongly acid cation exchanger for separation of monosaccharides e.g. xylose from magnesium sulfite cook liquor is also known from the publication WO 97/49658. The chromatographic separation has been carried out using a simulated moving bed. However, the separation of certain monosaccharides by using strong acid cation exchange resins has turned out to be difficult. According to Samuelson (Samuelson, O., Chromatography on ion-exchange resins, J. Methods Carbohyd. Chem. 6 (1972) 65-75), for instance, the separation of rhamnose from other carbohydrates with strong cation exchange resins has been possible by using solvents e.g. alcohol as an eluent. Rhamnose is eluted before most other carbohydrates because it has a shorter retention time than aldoses and ketoses when aqueous ethanol is used as eluent. Water would be a preferred eluent, but when it is used the problem is that the various carbohydrates, such as rhamnose, arabinose and/or xylose have the tendency to elute at almost similar retention time whereby the fractions will overlap. The separation has not been proposed to be done by water eluent.
The separation of carbohydrates, especially xylose by strong acid cation exchangers has been practised industrially but is complicated and succeeded only in one way. The method presented in U.S. Pat. No. 5,998,607 has been used especially for separating xylose from the magnesium spent liquor. The problem has been the unsufficient separation of xylose and xylonic acid. The use of a weakly acid cation exchange resin did not give any benefit when solving the problem. In the method the separation requires two steps. In the first step the cation exchanger resin is used preferably in alkaline earth form, more preferably in Mg2+ form and in the second step cation exchange resin is preferably in alkali-metal form (e.g. sodium). However, the separation of monosaccharides has also been found to be unsatisfactory since all the other monosaccharides elute at almost same retention time with xylose. The pH in the process was low. The resin in a divalent form seemed to separate the xylose more effectively than the resin in a monovalent form.
Publication PCT/FI00/00350 discloses sulphonated polymer resins, especially ion-exchange resins and the preparation of such resins. The polymer is a styrenedivinylbenzene copolymer, strongly acid cation exchange resin. The cross-linking agent can also be isoprene, allyl methylacrylate, vinyl methacrylate, glycol methacrylate or glycol diacrylate. According to the publication PCT/FI00/00350 the sulphonated polymer resin can be used as a chromatographic resin, ion exchange resin or as a catalyst resin.
U.S. Pat. No. 4,359,430 describes a process for recovering betaine from natural sources such as beet molasses, residue molasses and vinasses. The process uses a chromatographic column of strong acid cation exchange resin in alkali metal form, sodium being generally the preferred alkali metal. Water is used as eluent in the process. The process results in three fractions. The first fraction is a non sugar waste fraction, the second is a sugar containing fraction and the third fraction consists substantially of betaine.
Publication WO 96/10650 discloses a method for processing a beet derived sucrose containing solution to yield a sucrose enriched fraction and a fraction enriched with a second organic compound, especially such as betaine, inositol, raffinose, galactinol or serine and other amino acids. A strong acid cation exchanger preferably in sodium or potassium form is used for the separation of the fractions. From Finnish Patent No. 960 225 it is also known a method for fractioning of molasses by using a strong acid cation exchanger.
Anion exchange resins have been used for separating fructose from glucose. Y. Takasaki (Agr. Biol. Chem. 36 (1972) pages 2575-77) and B. Lindberg et al. (Carbohyd. Res. 5 (1967), pages 286-291) describe the use of an anion exchanger in bisulfite form for the separation of sugars. Water is used as eluent. However, the use of anion exchange resins does not result in a good xylose separation because the xylose is overlapping by other sugars. The separation of rhamnose has not been suggested. The separation of fructose and glucose by an anion exchanger in a bisulfite or sulfite form is known also from Patent FR-2 117 558.
U.S. Pat. No. 5,084,104 discloses a method for separation of xylose from pentose-rich solution, e.g. from birch wood. A chromatographic column which comprises a strong base anion exchange resin is used. The anion exchange resin is in sulfate form. Using this method xylose is retarded most strongly, but the other monosaccharides are eluted faster.
A method for preparing of L-arabinose is known from the publication WO 99/57326 where the process is charachterized by contacting plant fibers with an acid to hydrolyze the fibers under such conditions that the L-arabinose ingredients contained in the plant fibers are selectively obtained. U.S. Pat. No. 4,880,919 discloses a process for separating arabinose from mixtures of monosaccharides containing arabinose and other aldopentoses and aldohexoses by adsorption on sulfonated polystyrene divinyl benzene cross-linked ion exchange resins in with Ca2+ and NH4+ forms and desorpting the adsorbate with water. A process for production of crystalline L-arabinose is known from U.S. Pat. No. 4,816,078.
The preparation of arabinose is also known from the U.S. Pat. No. 4,664,718. In the method the arabinose is separated from the monosaccharide mixture containing also other aldopentoses and aldohexoses. The feed is contacted with calcium-Y-type or calcium-X-type zeolite and arabinose is adsorbed selectively. The desorption is conducted with water or ethanol.
Publication DE 3 545 107 describes a method for preparation of rhamnose from arabic gum. A strongly acid cation exchange resin is used for the separation of the sugar and rhamnose is purified by adsorption with an activated charcoal. Arabinose is also separated with this method.
Barker, S. A. et al (Carbohydrate Research, 26 (1973) 55-64) have described the use of poly(4-vinylbenzeneboronic acid) resins in the fractionation and interconversion of carbohydrates. In the method water is used as an eluent. The best yield of fructose was received when the pH was high. The resins have been used to displace the pseudo equilibrium established in aqueous alkali between D-glucose, D-fructose and D-mannose to yield D-fructose.
Surprisingly it has been found out that when using weakly acid cation exchange resins an improved chromatographic separation of carbohydrates is accessed. In addition to other features the order of separation seems to be affected by the hydrophobic/hydrophilic interactions of carbohydrates with resin and an improved separation of carbohydrates is resulted. Other commonly known features in chromatographic separation of carbohydrates on ion exchange resins include e.g. ion exclusion and size exclusion. If the resin is in the hydrophilic form the most hydrophobic monosaccharides seem to elute first and the most hydrophilic last. This results in a different elution order than previously found.